Acoustic device



L'. G. BosTwlcK Jan, 28, 1941.

ACOUSTIC DEVICE Filed Dec. 20, 1938 2 Sheets-Sheet 1 NOJ Y lo, F/G.3

.- /NVENTOR L. G. BOSTW/CK 5y v l O'ab. A TTORNEY Jn- 28, 1941... y L G, BGS-[WICK 2,230,104

ACQUSTIC DEVICE Filed Dec. 20, 1938 2 Sheets-Sheet 2 A TTORNEV atented Jan. 28, 1941 gisant ACQUISTI@ DEVICE lLee G. Bostwick, Chatham, N. J., assignor to Bell Telephone Laboratories Incorporated, New York, N. Y., a corporation of New York p Application December Ztl, 1938, Serial No. n24%,?92

17 Claims.

This invention relates to acoustic devices and more specifically to electroacoustic transducers suitable for two-way telephone conference systems. More particularly, it relates to certain relative arrangements of transmitter and loudspeaker means and to forms of transmitter means peculiarly adapted for such arrangements.

In loud-speaker two-way telephone conference systems, it has been found important to have large acoustic transmission loss between the loudspeaker and transmitter at each station. -If the loud-speaker and transmitter are located on the conference table, as seems desirable from several considerations, the transmitter should be equally sensitive to speech of all conferees around the table, while discriminating against sounds from the loud-speaker.

One object of this invention, therefore, is to insure good transmission and reception for each member' of a telephone conference group at each conference station.

A further object of this invention is to obtain a transmitter means having a desired polar response characteristic.

in accordance with one feature of this invention, the transducer unit comprises pressure gradient or velocity rtransmitter means and a loud-speaker located at a point of minimum transmitter sensitivity and oriented to radiate as much as possible away from the transmitter.

A further feature oi .the invention resides in a transducer unit constructed. in accordance with the foregoing feature in which the transmitter means comprises a plurality of transmitter elements arranged in a vertical line with their axes of maximum sensitivity in angular relation to each other, the outputs of one or more of the elements being modified as to magnitude and/or phase relation.

Another feature of this invention pertains to the use of ribbon-type velocity transmitter units for the transmitter elements.

An additional feature of this invention resides in a transmitter including two ribbon velocity transmitter units with vertically aligned ribbon diaphragms, having their planes angularly related, ancl means connected to the ribbon diaphragms to modify the outputs of the units as to phase and magnitude.

A further and more specific feature of this invention relates to a transmitter means employing two ribbon diaphragms in line vertically with the planes of the ribbon diaphragms at right angles and with the output voltage of one (ci. rvs-Qn ribbon unit shifted substantially 90 degrees in phase by an electrical network.

The invention and the foregoing and other features thereof Willbe understood more clearly and fully from the following detailed description with reference to the appended drawings,

in which:

Fig. l is a general perspective view of a trans-A ducer unit illustrative of this invention and a vportion of the table top on which it rests;

Fig. 2 is an elevational View of a transmitter constructed in accordance with this invention with its casing broken away to show the interior structure;

Fig. 3 is a section on line 3-3 of Fig. 2;

Fig. 4 is a perspective view of the transmitter shown in Fig. 2 with the casing removed and part of the supporting brackets broken away;

Fig. 5 is a wiring diagram showing how the transmittermay be connected to amplifier means v through a suitable network; and

Figs. 6 and 7 are sectional detail views showing how .the ribbon is clamped and one way of making electrical connections thereto.

Referring now to the drawings, in Fig. l, it) denotes the transducer unit comprising a transmitter lili and a loudspeaker HD2 resting on a conference table, a portion of which is shown at 93. As will be apparent from the drawings the speaker 92 radiates toward the table and its 30 sound output is distributed therearound by the arched or convex member itt. This arrangement insures a uniform distribution of sound around the loud-speaker in a generally horizontal direction.

If the transmitter lill lcomprises a single riboon velocity unit with the ribbon vertical, the foregoing arrangement gives minimum acoustic coupling between loud-speaker and transmitter. This is due to the wellalsnown low sensitivity of 0 this type of transmitter to sounds in the plane of its ribbon diaphragm. However, this characteristic directivity is objectionable since it does not permit uniform pick-up in a horizontal plane. This difficulty inherent in the single ribbon unit is overcome iri'accordance with one feature of this invention by employing a plurality of ribbon elements, vertically in line with the planes of the ribbon daphragms at angles with respect to each other, and means for modifying the phase and/or magnitude of the output of certain of the Aribbon units. Thus, various pick-up patterns in a horizontal plane may be provided. Such patternsmay be made generally elliptical for a long conference table or circular for a square or round 56 table. Other desired patterns may be formed by proper arrangement of the angles between the ribbon planes' and with suitable modification of the outp'ut of certain ribbon units as to phase and/or magnitude.

In the embodiment of the invention illustrat-v ed, the transmitter means comprises two ribbon diaphragms vertically in line with their planesA Referring again to the drawings and more par;A

ticularly Figs. 2, 3 and 4, I0 denotes a cylindrical casing or housing of brass or the like for enclosing the transmitter.' The ends of casing I9 are closed by two circular members and I2 of brass or other suitable material. The closure acts as a base for thec transmitter and has brackets I3 mounted thereon. The brackets |3 may be secured to the base by screws I4 or other suitable means. The member may be provided with a projecting bushing 42 for accommodation of the cable 4| and for attaching the transmitter to the loud-speaker casing. It is desirable under some conditions to employ material, such as soft rubber, in the connection between the transmitter and loud-speaker housings. Such an expedient inhibits the transference of mechanical vibrations between the housings.

'I'he casing Ill is perforated with a plurality of openings I5 for the passage of sound waves. A lining I6 of fabric, such as silk cloth, covers the openings -I5. The closures I| and I2 are grooved to receive theends of the casing I0 which are secured therein by soldering or like means. Suitable fastening means, suchvas screws I1, secure the casing I0 to the brackets I3.

The top lends of brackets I3 are turned inwardly, as shown, at I8, to provide supports for a mounting ring I9 of non-magnetic material, suchas brass. Bridging members 20 are dlametrally mounted, on opposite sides of the ring I9 and at right angles to each other. 'I'he lower bridge member 20 and ring I9 are fastened together and secured to portions I8 of the brackets I3 by screws 22. The upper bridge member I20 is attached to ring I9 by means of screws, one of which is shown at 23. The upper and lower transmitter units are mounted on upper and lower bridge members 20 respectively.

Inasmuch as the twoV transmitter units are of the same construction, the same reference characters are applied to like parts of each. For convenience of description the upper unit may be designated as No. 1 and the lower unit as No. 2. Each unit includes a magnetic structure comprising a pair of pole-pieces 24 of 'iron or the like and a magnet 25 of suitable magnetic material, such as an alloy comprising, cobalt 12 per cent, molybdenum 16 or 1'1 per cent and the remainder iron.

The pole-pieces 24 havel reduced portions 26, best seen in Figs. 2 and A4, and are mounted in parallel relation with the portions 24 in juxtaposition to dei'lne an elongated air-gap. Fastening means, such as screws 21, secure the pole-pieces to a bridge member 29. The magnet 25 is attached tothe pole-pieces 24 by tabs or plates l sa. The silver foil provides low uhmm' connect other end of the ribbon is clamped in a similar 38, 40 and 44 taken out in a shielded three-con- 23. The plates 28 may be'secured to the polepieces 24 by screws 29 and spot-welded to the magnet as at 30.

A ribbon diaphragm 3| is mounted in the air l, gap between pole-pieces 24. The ribbon-'..Jlsclamped at one end in a recess ofbridge*lnegn-l ber 20 and at the other end to -thes'upport'32 by a clamp 33 as shown in detail in Figs. 6 and-*1. 'Ihe support 32 is secured to the ends of portionsy 26 o! the pole-pieces -24 by screws 33.

7,., As shown in Fig. 6, the end of the lribbonthat, is attached to bridge member 20 is in relectrical connection therewith, being l `claiifillidf between" sheets. 53 ofsilver foil or the like, whihl-shee are in electrical contact with-members,

to the ribbon and also acts as a cushioninfgfmean to avoid damage to the ribbon in clamping. The

manner between sheets 5| of silver foil which are 20 insulated from members 32 and 33 by sheets 52 of insulation.

The ribbon 3| is preferably of aluminum foil and may be transversely corrugated near its ends only. The central portion of the ribbon may be 25 stiened by virtue of its curved cross-section as indicated in Fig. 3.

The ends ofthe ribbon, as illustrated in Fig. 5, are connected to the primary Winding of a transformer 34 by conductors 35 and 36. As shown in Figs. 2 and 7, both connections may be made adjacent one end of the ribbon. The connection from conductor 35 is to clamp 33 and then via the pole-pieces 24 to the grounded end of the ribbon at bridge member 20. The conductor 36 may be`connected to the ribbon through a tab 90, clamped against the insulated silver foil 5| by insulated screw 53, asl shown in Fig. 7. y

Connection to the external circuit may be made by conductors 31 and 38 for unit No. 1 and 40 by conductors 39 and 40 for unit No. 2. The conductors 31 and 39 may, for convenience, be spliced to a common conductor -44 at a point adjacent the lower transformer and conductors ductor cord 4| through the bushing 42.

The transformers 34 are mounted on the magnets 25 by suitable clamping brackets 43.

Referring now to the wiring diagram, Fig, 5, it will. be seen that each ribbon diaphragm 3| is connected bythe conductors 35 and 36 to the primary winding of a transformer 34. The secondary windingvin unit No. 2 is connected by conductors 49 and 39 and common conductor 44 to a phase shifting network, comprising inductor 45 and condenser 46. Conductors 38, 31 and 44 connect the transformer secondary winding in unit No. 1 to attenuating means such as resisto!l 41. 'I'he combined output is conducted to ampliiier 50 by wires 48 and 49. The attenuating means 41 is designed to substantially match the attenuation of the phase shiftingnetwork in order that the two outputs may be properly combined.

Inductor` 45 comprises twocoil portions, serially connected and inductively coupledy to each other with a tap between the two portions for the condenser connection. The inductance and capacitance of network elements 4'5 and 46 should be of such values that their impedances are substantially equal for a frequency intermediate the highest and lowest frequencies of the transmitter range. The'impedances of the inductor 45 and the condenser 46 are made small with 'Il respect to the impedance of the transmitter elements.

In a transmitter constructed in accordance with this invention, the output voltage of one ribbon diaphragm unit combined with that of the other after it has passed through the phase shifting network, is independent of the direction in the horizontal plane from which the energizing sound originates, as is shown by the following equations. The ribbon diaphragms, for convenience, may be designated as No. 1 and No. 2 in accordance with the unit designation of the prior description. If e1 represents the instantaneous voltage from ribbon No. 1 measured across resistor 4l and e2 the instantaneous voltage at the output of the phase shifting network which is connected to ribbon No. 2, as indicated in Fig. 5, then according to the known properties of the ribbon transmitter e1=E1 cos 0 cos wt e2=E2 cos (0 0) cos (wt-e) and lrl==the phase shift of the voltageof ribbon No. 2 introduced by the electrical network.

If a and fr are made 90 degrees Equation 2 becomes e2=Ez cos (9D- 0) cos (wt-90) :E2 sin 6 sin wt (3) Making E1=E2=E, the combined instantaneous voltage e for the two ribbons is then:

e=e1+e2=E1 cos 0 cos wt-l-Eav sin 0 sin zutY E cos (wt-0) (4) It is evident that the magnitude of the voltage represented by the coecient E in Equation 4 is independent of the angle 6 in the horizontal plane from which the sound originates. At the same time zenith and nadir sounds are discriminated against because each ribbon diaphragm is insentive to sound from a direction in the plane of the ribbon diaphragm and, therefore, the com? bined voltage of the two ribbon diaphragm units for these sounds is small. Thus, the polar sensitivity pattern in av horizontal plane is a circle while the pattern in 'any vertical plane through the ribbon is similar in form to a gure 8.

To obtain the above results ift is necessary that the voltage output of one ribbon be shifted 90 degrees in phase with respect to the other. Ii the two ribbons at right angles are connected in series without a phase shifting means, a polar sensitivity pattern similar to that for one ribbon is attained. The two transmitter voltages combine in a manner to cause insensitivity in a plane degrees to that of each of the two ribbon l planes.

The foregoing discussion of the operation ofl the transmitter assumes exactly 90 degrees phase -shiftin the electrical network. Such a constant phase shift withsmall and uniform insertion loss is diilicult to obtain over a-wide frequency range. It has been found, however, that a network such 75 as shown in the wiring diagram, Fig. 5, produces a close approximation. to that desired with tolerable loss values.

As heretofore stated, .other horizontal pick-up patterns may be obtained by suitable arrangements Aof the transmitting elements and the phase and magnitude modifyingmeans. For example, elliptical patterns with desired major and minor axes may be produced by several arrangements. One such arrangement comprises two sound responsive elements with 90-degree relative rotation, 90degree phase shift of one and unequal sensitivities. Another arrangement comprises two elements with i5-degree relative rotation, 45- degree phase shift of one and equal sensitivities. A third arrangement uses three elements, one with l5-degree and another with Q30-degree rotation, l-degree and 90-degree phase shifts, respectively, and equal sensitivities'.

Although specific embodiments of the invention 20 have been shown and described,l it is to be understood that they are illustrative and not limiting the scope of the invention as defined in the appended claims.

What is claimed is:

1. A loud-speaker-acoustic pick-up device comprising a unitary structure including a ysound pick-up means having an axis of low sensitivity, a loud-speaker located on said axis and oriented to radiate away from said pick-up means, and means associated with'said loud-speaker for distributing a substantial portion of its output over 'a plane perpendicular to said axis of low sensitivity.

2. A transducer unit comprising an acoustic transmitter capable of picking up sounds from all directions in a horizontal plane therethrough and substantially insensitive to nadir sounds, a

loud-speaker mounted beneath said transmitter pick-up means and electrical phase shiftingv means so constructed and arranged that the transmitter responds to sounds from all directions in a horizontal plane in accordance with a predetermined sound direction pick-up pattern, but is substantially insensitive to zenith and nadir sounds.

5. An acoustic transmitter including sound pick-up means and electrical phase shifting means, so constructed and arranged that the transmitter responds to sounds from all direcf tions in a horizontal plane, in approximately the samedegree, but is. substantially insensitive to zenith and nadir sounds. A

6. An acoustic transmitter comprising a plurality of pressure gradient transmitter units is sensitive to sound in all directions in a hori-f zontal plane in accordance with a predetermine ing conductive ribbon diaphragms, said diaphragms being axially in line and having their planes at predetermined angles, and means connected at one or more of said ribbons for modifying the phase and magnitude of its output voltage.

8. An acoustic transmitter comprising two pressure gradient transmitter units having conductive ribbon diaphragms, said diaphragms being axially in line and having their planes mutually perpendicular, and means connected to one ribbon to shift vthe phase of its output voltage substantially 90 degrees.

9. An acoustic transmitter including sound pick-up means and. electrical output modifying means so oriented and connected that the polar diagram of transmitter sensitivity to sound dire'ction in al horizontal plane through the pick-up means issubstantially elliptical and is substantially a gure 8 for any vertical plane through the pick-up means with maximum discrimination against zenith and nadir sounds.

10. An acoustic transmitter including sound pick-up means and electrical phase shifting means so oriented andconnected that the polar diagram of transmitter sensitivity to sound direction in a horizontal plane through the pick-up means is substantially circular and is substantially a iigure 8 for any vertical plane through the pick-up means with maximum discrimination against zenith and nadir sounds.

11. An acoustic transmitter comprising a tworibbon pressure gradient sound pick-up device, a phase shifting network and an attenuating means, the ribbons being axially in line with their planes nected to shift the output of one ribbon 90 degrees in phase, and the attenuating means connected and proportioned to reduce the magnitude of one ribbon output below that of the other, and means for combining the outputs of the two ribbons whereby said transmitter means responds substantialy equally to sound in an elliptical field extending in a. generally horizontal direction, but discriminates against zenith and na dir sounds.

12. An acoustic transmitter comprising two pressure gradient type transmitter units having ribbon diaphragms, said units being mounted on `a common support and so oriented that the ribbon diaphragms are axially in line with their planes mutually perpendicular, means responsive to the output of one unit to shift the phase of its voltage substantially 90 degrees with low loss, means responsive to the output of the other unit to introduce a loss substantially equivalent to that in the phase shifting means but without shifting the phase. of its voltageand means for combining y ,M l the moditled'outputsofthe two units.

mutually perpendicular, the network being con- 13. An acoustic transmitter as dened in claim 12' in which the phase shifting means lcomprises an inductor having two inductively coupled, serially connected winding portions, and a capacitor connected to the junction of said winding portions.

14. An acoustic transmitter including a tworibbon pressure gradient sound pick-up device and a phase shifting network, the ribbons being axially in line with their planes mutually perpendicular and the network being connected to shift the output of one ribbon substantially 90 degrees in phase, and means for combining the outputs of the two ribbons, whereby said transmitter responds substantially equally to soundsin a generally horizontal direction, but discriminates against zenith and nadir sounds.

15. An acoustic transmitter comprising sound pick-up means including two transmitter units having bi-directional sound pick-up characteristics, means connected tothe output of one of said vunits to shift the phase thereof substantially 90 degrees, and means for combining the shifted output of said one transmitter unit with the output of the other unit, said transmitter units being so oriented with respect to each other that the transmitter means is substantially equally responsive to sounds from any direction in or near a given plane through the sound pick-up means, but discriminates against sounds from a direction perpendicular to said plane and in line with the pick-up means.

16. An acoustic transmitter comprising two bidirectional transmitter units mounted on a common support with their axes of maximum sensitivity at right angles to each other, means for modifying the output of one of said transmitter units by shifting its phase substantially 90 degrees, and means for combining this modified output with that of the other transmitter unit, whereby said transmitter means is non-directional as to sounds in a horizontal plane passing therethrough butis substantialy insensitive to zenith and nadir sounds. A Y

17. An acoustic'transmitter comprising a supporting structure including a base, bracket means mounted on said base and having arms extending therefrom, a supporting ringI mounted on the extremities of said-arms, and two diametral bridge members attached to said ring on opposite sides pole-piece pair, and a. ribbon diaphragm mounted4 in each air-gap, said diaphragms being axially in line and with their planes mutually perpendicul'ar.k

LEE Q. Bos'TwIcK. 

